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Loop Start Signaling
November 2009 302
Loop Start Signaling
Brooktrout modules support two types of loop start signaling:
FXO Loop Start
FXS Loop Start
When operating in FXO mode, the module assumes the far end of the
connection is an FXS termination. When operating in FXS mode, the
module assumes the far end is an FXO termination.
FXS Loop Start is the protocol used for Brooktrout's analog modules.
The sequence of events and timings is the same for both analog or
digital line interfaces.
The timing diagrams in this section show the Receive and Transmit
Signaling Bits in reference to the digital signaling mode. The analog
signals are a direct mapping of the signaling bits.
In the FXS diagrams, the electric signals sent by the station are
represented in bit A: loop current presence (off-hook) is represented
by setting bit A high (one); absence (on-hook) by bit A low (zero). Bit
B does not change. The signals received by the station, ring signal
and loop current feed (battery), are represented using both A and B
bits: loop current feed is represented by setting bit A low (zero);
absence by bit A high (one). Presence of ring signal is represented by
setting bit B low (zero); absence by bit B high (one).
The FXO diagrams are the mirror-image of the FXS diagrams. The
signals sent by the module (FXO) toward the network (FXS) are the
same that the station (FXS) received from the network (FXO) on the
FXS diagrams.